Recently, disk recording/reproducing apparatuses have been used for many applications, such as for DVDs, MDs, CDs, CD-ROMs, or the like, which have been diversified year after year. In addition, they have been increasingly improved in density, performance, quality, and added value and have been reduced in size. Particularly, in disk recording/reproducing apparatuses using recordable magneto-optical media, the demand for those used for data and music recording tends to increase considerably, and further reduction in size and thickness and further improvement in performance and recording density have been asked for.
Conventionally, many techniques relating to optical heads for magneto-optical disks have been reported.
As an example of a conventional disk recording/reproducing apparatus, an optical head for a magneto-optical disk is described with reference to drawings as follows.
FIG. 23 is an exploded perspective view showing a schematic configuration of a conventional optical head, and FIG. 24 is an enlarged sectional view showing a method of fixing a reflecting mirror in the optical head shown in FIG. 23. FIG. 25A is an optical-path diagram showing optical paths in the optical head shown in FIG. 23, and FIG. 25B is a plan view showing light receiving areas and light spots on a light receiving plane of a multisplit photodetector. FIG. 26 is a signal circuit diagram showing a method of processing signals obtained by the multisplit photodetector in the optical head shown in FIG. 23.
In FIGS. 23 to 26, numeral 101 indicates a semiconductor laser, numeral 102 a collimator lens, numeral 103 a diffraction grating, numeral 104 a composite device including a beam splitter 104a, a polarization separation element 104b, and a return mirror 104c, numeral 105 an objective lens, numeral 106 an information recording medium (a magneto-optical disk) having a magneto-optical effect, numeral 107 receiving optics for monitoring, numeral 108 a convex lens, numeral 109 a concave cylindrical lens, numeral 110 a holding member, numeral 111 a multisplit photodetector, numerals 112 and 113 focal points of light spots, numeral 114 a main beam (P-polarized light) formed on the multisplit photodetector 111, numeral 115 a main beam (S-polarized light) formed on the multisplit photodetector 111, numeral 116 a main beam (P+S-polarized light) formed on the multisplit photodetector 111, numeral 117 light spots formed of preceding beams out of sub-beams, and numeral 118 light spots formed of subsequent beams out of the sub-beams. Numeral 119 denotes a four-split light receiving area, numeral 120 a preceding-beam receiving area, numeral 121 a subsequent-beam receiving area, numerals 122a and 122b are light receiving areas for an information signal, numeral 123 subtractors, numeral 124 adders. Numeral 125 represents an optical bench, numeral 126 a reflecting mirror, numeral 127 adhesion reference planes of the optical bench 125, numeral 128 an adhesion reference plane of the reflecting mirror 126, numeral 129 positioning walls for positioning the reflecting mirror 126, which are formed in the optical bench 125, numeral 130 adhesive storages, numeral 131 a UV adhesive, and numeral 132 an objective lens actuator.
The conventional optical head with the above-mentioned configuration is described as follows.
The reflecting mirror 126 is fixed to the optical bench 125 as follows. As shown in FIGS. 23 and 24, the positioning walls 129 for positioning the reflecting mirror 126 are formed in the optical bench 125. The reflecting mirror 126 is mounted along the positioning walls 129. After that, as shown in FIG. 24, by applying a preload 151 in a direction parallel to a reflecting plane of the reflecting mirror 126 and a preload 152 in a direction perpendicular to the reflecting plane of the mirror 126, one side face (a lower end face) of the reflecting mirror 126 and a positioning wall 129 of the optical bench 125, and the adhesion reference plane (the plane opposite to the reflecting plane) 128 of the reflecting mirror 126 and the adhesion reference planes 127 of the optical bench 125 are brought into contact, respectively. Thus, the reflecting mirror 126 is positioned with high precision. In this state, the UV adhesive 131 is applied to the adhesive storages 130 and ultraviolet rays are irradiated thereonto, thus bonding and fixing the reflecting mirror 126 to the optical bench 125 through the UV adhesive 131 with high precision.
Next, the following description is directed to the operation of an optical head in a completed state, into which various components have been incorporated.
A beam emitted from the semiconductor laser 101 is converted to parallel light by the collimator lens 102 and separated into a plurality of different parallel beams of light by the diffraction grating 103. The plurality of different parallel beams of light pass through the beam splitter 104a in the composite device 104 and then form a light spot of a main beam with a diameter of about 1 micron and respective light spots of the preceding beams and the subsequent beams as the sub-beams in a so-called “three beams method” on the information recording medium 106 by the objective lens 105 incorporated into the objective lens actuator 132. The respective light spots of the preceding beams and the subsequent beams are formed in front of and behind the light spot of the main beam at certain intervals on the same track as that on which the light spot of the main beam is formed. A parallel beam of light reflected by the beam splitter 104a in the composite device 104 enters the receiving optics 107 for monitoring to control a driving current for the semiconductor laser 101.
Reflected light from the information recording medium 106 travels along the reverse path to be reflected and separated by the beam splitter 104a in the composite device 104, which is then incident on the polarization separation element 104b. The semiconductor laser 101 is mounted so that the polarization direction of a beam emitted therefrom is parallel to the surface of the paper showing FIG. 25A. Incident light on the polarization separation element 104b is separated into three beams of light, i.e. two beams of light whose polarized components are orthogonal to each other and one beam of light having two polarized components orthogonal to each other, by the polarization separation element 104b. These three beams of light are then reflected by the reflecting mirror 104c. 
Reflected light that has passed through the composite element 104 enters the convex lens 108 with an approximate cylindrical shape to become convergent light and then enters the concave cylindrical lens 109 with an approximate cylindrical shape. In this case, the concave cylindrical lens 109 is provided so as to have a lens effect in the direction that forms an angle of about 45 degrees with respect to an image on a recording track of the information recording medium 106 oriented in the direction of W1 in the present example.
Light that has passed through the concave cylindrical lens 109 generates astigmatism that serves as a means for detecting a focus error signal. When passing through a plane having no lens effect in the concave cylindrical lens 109, the light travels in the optical path indicated with an unbroken line to be converged at the focal point 112. When passing through a plane having a lens effect, the light travels in the optical path indicated with a broken line to be converged at the focal point 113.
The concave cylindrical lens 109 is rotated to be adjusted so that the direction W2 (not shown in the figures) having a lens effect in the concave cylindrical lens 109 forms an angle of about 45 degrees with respect to the holding member 110. In addition, the convex lens 108 and the concave cylindrical lens 109 are fixed by the holding member 110 at a predetermined interval in an optical axis direction.
The multisplit photodetector 111 is mounted so that its light receiving plane is positioned approximately midway between the focal point 112 and the focal point 113. The sums of the electric signals generated in respective diagonal areas in the four-split light receiving area 119 at the center are calculated and one sum is subtracted from the other sum. Thus, a focus error signal is detected by a so-called “astigmatism method”. The difference between the light spots 117 formed of preceding beams and the light spots 118 formed of subsequent beams is calculated, thus detecting a tracking error detection signal by a so-called “three beams method”. By calculating the difference between the main beam 114 composed of P-polarized light and the main beam 115 composed of S-polarized light, an information signal of the information recording medium can be detected by a differential detection method. Furthermore, by calculating the sum of them, a prepit signal can be detected.
However, in the above-mentioned conventional configuration, the positioning walls 129 are provided for bonding and fixing the reflecting mirror 126 to the optical bench 125 with high position precision. Therefore, the overall height of the optical bench 125 increases. As a result, the overall height of the optical head increases.
Individual optical benches 125 may be different and therefore the variance in angle of the adhesion reference planes 127 with respect to the reference plane of the optical bench 125 occurs. Consequently, an axis of light entering the objective lens 105 varies considerably, resulting in unstable performance of the optical head.
On the other hand, when it is sought to process or form the adhesion reference planes 127 with high precision with respect to the reference plane of the optical bench 125 in order to reduce the variance in angle of the optical axis of the optical head, the cost for processing or forming the optical bench 125 increases.
The UV adhesive 131 may expand or contract depending on the variation in thermal environment in some cases. In the conventional configuration, the UV adhesive 131 fills the adhesive storages 130 and adheres to a part of the back face of the reflecting mirror 126. Therefore, the expansion or contraction of the UV adhesive 131 changes the mounting angle of the reflecting mirror 126 slightly. As a result, the optical axis of the optical head varies and therefore the performance of the optical head is deteriorated, which also has been a problem.
Since a working accuracy is required in the step of bonding and fixing the reflecting mirror 126 to the optical bench 125 with the UV adhesive 131, too much time and cost are required and thus mass-productivity is decreased, which has been a problem.